The present invention relates to an apparatus for decomposing plastic waste so as to recover it as fuel oil, fuel gas or the like.
In recent years, how to perform the waste disposal has been a very important problem. In particular, plastic waste, when subjected to incineration, produces high temperatures to damage an incinerator so that the plastic waste is considered as waste whose disposal is difficult and a disposal method thereof has been sought. Most of the plastic waste, such as polyethylene or polystyrene, is formed into a liquid phase polymer through a thermal decomposition treatment and, when subjected to further thermal decomposition, it can be recovered as fuel oil or fuel gas. Since it is the best for the waste disposal to recover those useful things from the waste, the development of apparatuses has been expected which can recover fuel oil or fuel gas from the plastic waste at a reasonable cost.
Various researches relating to plastic waste decomposing apparatuses have been made from the foregoing aspect, and techniques of, for example, Laid-open Patent Publication No. 4-180878, Laid-open Patent Publication No. 5-237645, Laid-open Patent Publication No. 5-263079 and so on have been proposed.
However, the fact is that those techniques have not yet reached the real stage of practical use. This is because there have been the following problems peculiar to those techniques. Specifically, decomposition of plastic waste is carried out by heating a liquid phase polymer to destroy a high-order structure of the polymer into a low-order structure, further heating it to produce vapor components depending on decomposition temperatures, and cooling them to obtain fuel oil or fuel gas of various kinds. At this time, since it is difficult to quickly separate from the liquid phase polymer the vapor components of low molecular weight produced through decomposition of the liquid phase polymer, there is a case wherein the vapor components are excessively heated at contact portions with a heating wall surface so as to produce much carbon. Since the produced carbon covers the heating surface to function as a heat insulating material, it makes difficult a control upon carrying out decomposition of the liquid phase polymer, particularly a decomposition temperature control, and increases the cost of running the decomposing apparatus. Further, if the carbon is entrapped into the recovered oil, the oil quality is lowered. There is a further problem that due to difficulty in keeping uniform the temperature of the whole liquid phase polymer upon performing the decomposition reaction, it is difficult to control the degree of the liquid phase polymer decomposition so that difficulty is encountered in selectively obtaining a recovery of a desired composition.
Therefore, an object of the present invention is to provide a plastic waste decomposing apparatus which is capable of quickly and efficiently separating from a liquid phase polymer a vapor component produced from the liquid phase polymer so as to suppress generation of carbon following a decomposition reaction, and further capable of performing the decomposition reaction in the state where the temperature of the whole liquid phase polymer is uniform, so as to efficiently recover the vapor component of a desired molecular weight.
For solving the foregoing problems, a plastic waste decomposing apparatus according to the present invention is basically a plastic waste decomposing apparatus comprising a decomposing chamber which is supplied with a liquid phase polymer, obtained by heating plastic waste, from its upper side to its lower side, and which is heatable from exterior thereof, wherein the liquid phase polymer is subjected to thermal decomposition while being agitated in the decomposing chamber to produce a vapor component which becomes fuel oil or fuel gas. The decomposing chamber comprises an upright cylinder including therein a rotator having a shape substantially corresponding to a shape of an inner periphery of the decomposing chamber and a helical screw provided on an outer periphery of the rotator, so that the liquid phase polymer flowing down is subjected to the thermal decomposition while being agitated to be forced upward by the screw in a reaction space formed between the inner periphery of the decomposing chamber and the outer periphery of the rotator.
In the plastic waste decomposing apparatus of the present invention, the reaction space is formed between the inner periphery of the decomposing chamber and the outer periphery of the rotator by providing, in the decomposing chamber in the form of the upright cylinder, the rotator having the shape corresponding to the shape of the inner periphery of the decomposing chamber. Specifically, in the decomposing apparatus of the present invention, by forming narrow the reaction space where the decomposition reaction of the liquid phase polymer is carried out, it is possible to allow the whole of the liquid phase polymer to approach the inner periphery of the decomposing chamber where the polymer is heated, so that the heating thereof becomes uniform. Therefore, in the decomposing apparatus of the present invention, since the liquid phase polymer can be heated in the state where the temperature gradient thereof is as small as possible, the selective vapor component recovery and the carbon generation control are made possible.
Further, the helical screw is provided on the outer periphery of the rotator in the plastic waste decomposing apparatus so that the liquid phase polymer flowing down in the reaction space is agitated to be forced upward by the screw. By subjecting the liquid phase polymer to such agitation, the vapor component produced in the liquid phase polymer is forcibly raised to the liquid level of the liquid phase polymer so that gas-liquid separation is facilitated. Therefore, in the plastic waste decomposing apparatus, it can be prevented that the vapor component entrapped in the liquid phase polymer is overheated to produce carbon. Further, the foregoing agitation of the liquid phase polymer provides a sufficient time for reaction of the liquid phase polymer and allows the high-viscosity liquid phase polymer, which has not yet produced the vapor component, to stay in the reaction space preponderantly. Accordingly, the rate of the liquid phase polymer just before producing the vapor component can be increased in the reaction space so as to increase the decomposition reaction rate while preventing the carbon generation.
It is preferable that the rotator in the plastic waste decomposing apparatus of the present invention is provided with a cavity which can be supplied with high-temperature gas. With this arrangement, the liquid phase polymer can be heated from two surfaces forming the reaction space, i.e. from both the outer periphery of the rotator and the inner periphery of the decomposing chamber, so that the temperature gradient in the liquid phase polymer can be further reduced and, by supplying greater heat quantity, the plastic waste decomposition amount per unit time can be increased.
Further, in the plastic waste decomposing apparatus of the present invention, it is preferable that a lower part of the decomposing chamber is formed into a conical shape, and a circulation pipe is provided for circulating the liquid phase polymer, discharged from a lower end of the decomposing chamber, into the decomposing chamber. It is possible that impurities which can not be decomposed by heating are entrapped into the plastic waste. In this case, it is necessary to remove the residue produced in the process of the decomposition. By tapering the lower part of the decomposing chamber so as to form the inner periphery thereof to have the conical shape, the residue is naturally collected and accumulated at the lower end of the conical inner periphery so that the removing operation thereof can be easily carried out. The removal of the residue can be performed by providing a means for discharging the residue, for example, in the form of a screw conveyor within the circulation pipe, or a means for discharging the residue at the lower end of the conical lower part of the decomposing chamber. In either case, if the residue is once collected at the lower end of the decomposing chamber, the removal process is facilitated. The means for discharging the residue may be adequately provided depending on kinds of polymer, residue and the like. When removing the residue as described above, it is possible that the high-viscosity liquid phase polymer, which has sunk to the lower end of the decomposing chamber upon discharging the residue, also flows out from the decomposing chamber. In this case, by supplying again the high-viscosity liquid phase polymer, having flowed out from the lower part of the reaction chamber, to the decomposing chamber via the circulation pipe, a loss of the liquid phase polymer is prevented thereby to increase the vapor component amount to be recovered. Further, by the circulation of the liquid phase polymer, a staying time of the liquid phase polymer in the reaction chamber can be ensured.